The long-term objective of this application is to enhance our understanding of the articulatory origins and perceptual motivations underlying the coarticulation of consonant and vowel segments in natural speech. This application joins together two research programs that study: 1) the acoustic characterizations of coarticulation using the locus equation paradigm; and 2) the articulatory underpinnings of coarticulation using the APEX vocal tract model. An integrated set of studies is described, which aim at uncovering the fundamental processes out of which emerge the regularities captured by locus equations. Data will include: 1) acoustic measurements, 2) articulatory configurations from an X-ray database, 3) APEX simulations of speech production based on a human speaker. The study will be test two theoretical hypotheses of coarticulatory behavior - a co-production/superposition account (Ohman, 1967) versus a more traditional phoneme-by-phoneme account first proposed by Joos (1948).